Design of the high field side antenna of the new reflectometric system for plasma position estimate in RFXmod2

A reflectometric system will be installed in the RFX-mod2 experiment, consisting of 4 couples of transmitting/receiving antennas working in the range 16-26.5 GHz in X-mode wave propagation for tokamak discharges. They will be placed within dedicated plasma accesses in the same poloidal section at 4 equispaced poloidal positions, two on the equatorial plane, High Field Side (HFS)/Low Field Side (LFS), and two at the vertical top/bottom ports. This configuration was conceived to perform plasma position control experiments without using the magnetic measurement signals. While the accesses in LFS, top and bottom positions will accommodate pyramidal antennas, the strict room constraints in the HFS position required a special routing of the feeding waveguide and the design of a different type of antenna, described in the paper. The horn reflector (also named hoghorn) type was preferred which allows radiating (and receiving) a beam at a 90 degrees direction with respect to the horn axis, which will be perpendicular to the equatorial plane. After fixing a reference working frequency f = 21 GHz (wavelength lambda = 14.3 mm), an antenna fitting the available room was designed by means of the COMSOL Multiphysics Radio Frequency module. Four different versions were developed by introducing some modifications of the aperture shape to study their effect on the antenna performance. FEM analyses were run for frequencies in the 17-26 GHz interval to characterize the frequency response in terms of radiative patterns of the total and far electric field. The directivity of the antennae was also evaluated. The 4 versions exhibited comparable responses and the observed beam directional properties at the expected plasma distance were considered acceptable for the development of this application. A prototype of the antenna has been realized by additive manufacturing process.